A human observer can view a scene under a variety of different illuminants, yet still see the same range of colors. For example, a white piece of paper remains resolutely white independent of the color of light under which it is viewed. This psycho-visual effect is referred to as “color constancy.”
In contrast, color imaging systems are less color constant in that they will often infer the color of the scene illuminant incorrectly. For example, a digital camera captures an image of an indoor scene under a reddish tungsten illumination. Since the light entering the camera is this tungsten illumination reflected from the objects in the scene, all objects in the scene will be recorded as redder than they would be had the illuminant been a whitish fluorescent light. This change in color is at odds with what a human observes (the human perception of the colors of the objects is relatively stable, even if the illuminant is switched between tungsten and fluorescent).
Color appearance models can be used to ensure color constancy in the images produced by the imaging systems. That is, the color appearance models can ensure that the colors in the scene appear the same regardless of the illumination.
However, these models depend upon knowledge of the white point of the image scene. If the imaging system does not measure the white point directly, the white point is estimated.